It all depends on the size of the image. You need to set your top edge and left edge to 50%, then subtract half of the content size. So for example, my image below has a width of 300px, so I subtract 150px from the left edge by setting the left margin and do the same operation on the top margin.
<!DOCTYPE html> <html> <body> <div style="position:absolute;top:50%;margin-top:-179px;left:50%;margin-left:-150px;"> <img src="image.png" width="300" height="358"> </div> </body> </html>